In the drilling of wells, such as oil and gas wells, both on land and at sea, it is the practice to set a length of relatively large well bore casing in the upper end section of the well bore, and subsequent drilling operations are performed through such surface casing. In certain offshore drilling operations, such as those conducted from a vessel or barge, the drilling operations are conducted through a base which is set and anchored on the floor of the water.
The bore hole may be quite large and the casing quite large, say, on the order of 54 inches or 60 inches. It is sometimes desirable to run the casing into the borehole closely behind the drill bit to prevent the sidewall of the earth from caving in or sloughing off and possibly sticking the drill pipe, or rendering very difficult, if not impossible, the forcing of casing into the hole. Similar problems may be encountered when large piles are being set in boreholes rather than being driven by a pile driver.
It has been proposed, for example, as disclosed in U.S. Pat. No. 3,123,160, granted Mar. 3, 1964, to interconnect with the casing in a well a retrievable drilling apparatus to produce a hole in advance of the casing as the casing is being lowered. The casing could be rotated to effect the drilling operation or the drilling device could be a fluid driven motor and drill combination.